The prior art contains numerous automobile radiotelephone designs. Likewise, the use of a wireless ultrasonic link in communications systems is known in the art. Examples of ultrasonic communications systems include U.S. Pat. No. 4,969,180 to Watterson, et al. and U.S. Pat. No. 4,542,262 to Ruff. Attempts to combine radiotelephone technology with wireless ultrasonic technology, however, have met with limited success, in part, because they have been based on half duplex systems that prevent simultaneous transmission between the parties. Further, there are also distortion problems and the space constraints of the passenger compartment that must be addressed.
One attempt to resolve these problems is found in U.S. Pat. No. 4,291,411 to Muller, et al. Muller connected the base unit of a radiotelephone to a secondary unit mounted in the back range of the interior of an automobile which communicated with a handset via either a half duplex infrared or ultrasonic link. The Muller design, however, imposes limits on user position and on the clarity of transmission because of the location of this secondary unit. The Muller design also prohibits simultaneous communication by the use of a half duplex system. Further, infrared transmission has proved to be less desirable than ultrasonic transmission in an automobile because it cannot be used in a full duplex system and because of distortion caused by high passenger compartment temperatures.
Other drawbacks with current radiotelephone systems include the inability of more than one passenger to participate in the conversation without having to pass the handset. Attempts to resolve this problem through the use of a loudspeaker/microphone system are ineffective due to the fact that only one of the calling parties may speak at a time and the inherent distortion level present with respect to those passengers in the rear of the passenger compartment. The ability to hear or speak over engine, road and wind noise is obviously limited.
Given that the clarity of an ultrasonic link depends largely on the placement of the secondary unit with respect to the handset, it would be desirable to provide for a wireless car radiotelephone handset which can operate distortion free from any position in the automobile in full duplex mode. Similarly, the use of multiple handsets would permit simultaneous multiparty party conversation and eliminate the distortion usually present with the use of a loudspeaker/microphone system.